1. Field of the Invention
The present invention relates to a solid-state imaging element such as a complementary metal oxide semiconductor device (CMOS) image sensor, a driving method therefor, and a camera system.
2. Description of the Related Art
In recent years, CMOS image sensors have attracted attention as solid-state imaging elements (image sensors) replacing charge coupled device (CCD) image sensors.
CMOS image sensors have overcome the following problems:
The manufacture of CCD pixels involves special processes, and a plurality of power supply voltages are used to operate them. In addition, it is necessary to operate a plurality of peripheral integrated circuits (ICs) in combination.
Such CCD technology increases system complexity. These various problems have been overcome by the CMOS image sensors.
X-Y addressing solid-state imaging elements such as the CMOS image sensors are configured to perform pixel selection row-by-row, column-by-column, or pixel-by-pixel on a pixel array unit having a two-dimensional array of pixels including photoelectric conversion elements which are arranged in a matrix.
Further, row-access solid-state imaging elements provide read access to the pixel array unit and reset access for shutter release.
CMOS image sensors generally employ a pixel reset method in which pixels are sequentially reset row-by-row. This method is called the rolling shutter method.
In the solid-state imaging elements, an optically masked image, or an optical black region, is also formed to determine the noise floor.
In the solid-state imaging elements, a technique for specifying a desired region in the pixel array unit and providing partial cutting and readout of pixel information concerning the desired region has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2001-45383).
Accordingly, a solid-state imaging element disclosed in Japanese Unexamined Patent Application Publication No. 2001-45383 has a function for cutting and reading out a necessary region (“window readout” function).
With the window readout, the solid-state imaging element can reduce the number of output lines to outside the solid-state imaging element.
In a solid-state imaging element, however, when a desired partial readout region is sequentially read row-by-row, the following phenomenon, called blooming, occurs because of no access control such as readout of pixels in rows below and above the partial readout region.
Charge photoelectrically converted and accumulated in a photoelectric conversion element in a pixel may exceed the capacity of the photoelectric conversion element and overflow to neighboring pixels.
The charge overflowing to the neighboring pixels affects the imaging performed by the neighboring pixels. In CMOS image sensors, an overflow of charge from pixels above and below the pixels in the partial readout region may cause a blooming phenomenon which produces unwanted image defects in this region.
One of methods for preventing blooming is as follows.
A window region is set larger than desired, and the set region is normally accessed to prevent unwanted flow of charge.
In this method, however, pixel data in unnecessary rows is output to the outside, resulting in an increase in the number of outputs to outside as shown in FIG. 1.
The output rows and the like of a row-access solid-state imaging element will now be described with reference to FIGS. 1 and 2.
As shown in FIG. 1, output rows of a row-access solid-state imaging element include rows in a recording pixel region, a color-processing margin region, an ineffective region, an optical black region, an ineffective optical black region, and a blanking region.
Data for the ineffective region, the ineffective optical black region, and the blanking region are unnecessary.
A blanking output is provided for the communication period between the solid-state imaging element and the outside, the waiting time for stabilizing the internal circuit after the settings of the solid-state imaging element, such as the gain value, have been changed, the timing adjustment between the solid-state imaging element and the outside, and the like.
In a blanking period, as shown in FIG. 2, no read access or reset access to the pixel array unit is performed. Alternatively, access to a specific address that does not affect readout data is performed.
In a solid-state imaging element, generally, no window region is set in an optical black region and the number of output rows is fixed by the pixel array unit.
Furthermore, the number of optical black lines depends on use. For example, a large number of optical black lines are necessary for photographing still images while a reduced number of optical black lines can be used for photographing moving images.
A method for preventing the occurrence of blooming and preventing an unnecessary row of pixel data from being output to the outside is disclosed in Japanese Unexamined Patent Application Publication No. 2006-310932.